The present invention is directed to a frequency analyzer in a planar waveguide technology which analyzer has a modulator-optical part having at least one planar waveguide modulator for modulating light travelling in a planar waveguide on a substrate and a lens optical part which is disposed in a position following the modulator part and including at least one lens for forming a Fourier transformation of the modulated light such as a geodesic lens on a substrate. The invention also is directed to a method of manufacturing a planar, geodesic lens on a substrate.
Acousto-optical frequency analyzers can be utilized for the frequency analysis of high frequency signals. These analyzers enable a frequency analysis in "real time" in a simple manner. The employment of an acousto-optical frequency analyzer in planar waveguide technology is of particular interest and is discussed by M. K. Barnoski et al. "Integrated Optic Spectrum Analyzer" IEEE Transactions on Circuits and Systems, Vol. CAS-26 No. 12, December 1979, pp. 1113-1124. Because of its compact structure, this technology offers the advantage of mechanical stability which is extremely important in devices for optically coherent signal processing. Moreover the planar format allows the employment of cost favorable manufacturing methods such as for example using photolithography.
Herebefore, the most frequently utilized manner of realizing a planar frequency analyzer provides a substrate of a monocrystalline LiNbO.sub.3 and an optical waveguide is formed in the substrate by diffusion of Ti therein. The light, which is from a source such as a gas laser or a semi-conductor laser, is coupled into the waveguide at one end face and after travelling along the waveguide strikes a photo detector row, which is coupled to the opposite end face of the waveguide. The waveguide modulator of the modulator optical part comprises an electro-acoustical transducer, which can be photolithographically applied to the substrate and which serves to generate a surface acoustical wave. The lens optical art, which preferably comprises a geodesic lens for the Fourier transformation, is positioned on the substrate in a position following the waveguide modulator. In addition, a geodesic lens for the collimation of light from the light source is also preferably supplied before the modulator i.e. between the modulator and the light source. The lenses can be produced in the substrate by means of ultrasonic erosion or with a diamond turning or milling method. Both these methods are known and examples are discussed by B. Chen, et al. "Diffraction-Limited Geodesic Lens for Integrated Optic Circuit," IEEE Journal of Quantum Electronics, Vol. QE-15, NO. 9 (1979), pp. 853-860 and D. Mergerian et al., "Diamond-Turned Aspheric Geodesic Waveguide Lenses in Lithium Niobate," Integrated Guided-Wave Optics Technical Digest, Incline Village, Nev., U.S.A., 1980, ME4.
There are still numerous problems in the manufacture of these lenses. In principal, a plurality of lenses can be manufactured in the ultrasonic erosion method with a high-precision manufactured tool. However, the wear of the tool proved to be too high in order to be able to reproducably manufacture a geodesic structure. In contrast thereto, the diamond turning or milling method seems to supply reproducable results but each lens must be individually manufactured with the same cost and must also be manually burnished or polished. Thus, the production of these lenses by the diamond turning method is an expensive procedure.